Condensation particle counter

ABSTRACT

Provided is a condensation particle counter. The condensation particle counter measures the number of ultra-fine particles by growing the ultra-fine particles through a condensing process. The counter comprises a capillary in which vapor of operating liquid is condensed and the ultra-fine particles grow. An insulating material surrounds the capillary to shut out heat flow between the capillary and the environment. The condensation particle counter according to the present invention can use various operating liquids including alcohol and water, and can be also applied to semiconductor clean rooms.

TECHNICAL FIELD

[0001] The present invention relates to a particle counter, and moreparticularly, to a condensation particle counter which measures thenumber of ultra-fine particles by growing the ultra-fine particleshaving their sizes of 0.1 μm or less which are used as condensationnuclei.

BACKGROUND ART

[0002] Measurement of the number of ultra-fine particles is prerequisiteto fundamental study of the particles for air pollution measurement, andhas also been applied to investigation of the cause why the ultra-fineparticles have been generated in semiconductor clean rooms and the likeso that the clean rooms can be kept at a clean state by removing theultra-fine particles from the clean rooms. As well known in the art, anoptical instrument such as a laser is utilized to measure the number ofthe ultra-fine particles. In general, a particle measurement limit ofthe optical instrument corresponds to about 0.1 μm in particle size.Thus, a condensation particle counter has been utilized for measuringthe ultra-fine particles having their sizes of 0.1 μm or less beyondsuch a measurement limit. The principle of the condensation particlecounter is that a liquid is condensed around the ultra-fine particles byutilizing the ultra-fine particles as condensation nuclei, and then, theultra-fine particles are caused to grow to such a degree that they canbe measured through the optical instrument.

[0003] In order to cause the liquid to be condensed around theultra-fine particles, the following three types of technologies arecurrently utilized. The first one is an oldest technology. According tothis technology, particles to be measured are injected into a containerwith water contained therein, the container is hermetically closed, andthen, inside pressure of the container is rapidly reduced. Thus, innertemperature of the container is rapidly decreased, and temperature ofwater vapor within the container is consequently lowered. As a result,the vapor becomes supersaturated. The vapor starts to be condensed undersuch a supersaturated state, and is then condensed around the particlesserving as condensation nuclei. After the condensation of the vapor hasbeen completed, the particles become water droplets that the particlesare enclosed in the condensed water. The particles can be easilymeasured through the simple optical instrument, because these waterdroplets are very large. However, since the measurement of the particlesaccording to a conventional condensation particle counter in which theparticles grow by such an expansion process should be intermittentlymade, there is a problem in that continuous measurement of the particlesis greatly restricted. Accordingly, this technology has been hardlyemployed at present.

[0004] According to the second technology, hot air with saturated watervapor and cold air with the particles are mixed with each other. Thus,supersaturated vapor is formed in a region where the hot and cold air ismixed. Even in such a case, the supersaturated vapor is also condensedaround the particles serving as condensation nuclei in the same way asthe first technology. Such a type of condensation particle counter iscalled a mixing type condensation particle counter. However, very highsupersaturation may be formed partly in the mixing type condensationparticle counter, and thus, the vapor is spontaneously condensed intothe water droplets even though the particles used for the condensationnuclei are not provided therein. Therefore, there is also a problem inthat the measurement of the number of the particles is inaccurate.Accordingly, this technology has been utilized only in some restrictedfields.

[0005] The third technology is a conductive cooling type condensationparticle counter of which constitution is shown in FIG. 1. Theconstitution of the conductive cooling type condensation particlecounter will be explained with reference to FIG. 1. Alcohol 12 iscontained in a storage pool 10, and a cylindrical absorbing member 22 isattached to an inner wall of a saturator 20 which is integrally formedwith and extended from the storage pool 10. The alcohol 12 is absorbedinto the absorbing member 22 which is made of porous material such asnonwoven fabric and of which one end 22 a is immersed into the alcoholwithin the storage pool 10, and thus, the other end 22 b of theabsorbing member is caused to be wetted by means of a capillaryphenomenon. At an outer wall of the saturator 20 is installed a heater26 for heating the alcohol permeated into the absorbing member 22 toabout 35° C. A condenser 30 is located downstream of the saturator 20and is provided with a thermo-electric cooler 32 which causes thecondenser 30 to be kept at a temperature of about 10° C. for condensingalcohol vapor. In order to sense and measure the grown particles, awell-known optical instrument 50, which comprises an assembly of mirrorsor lenses and utilizes a laser or a semiconductor laser as a lightsource, is located in the vicinity of a leading end of the condenser 30.Further, a flowmeter 60 for regulating a flow rate of the grownparticles by means of opening/closing operation of a valve (not shown)and a vacuum pump 70 for sucking the grown particles thereinto aresuccessively installed downstream of the condenser 30 in a state where apipe 62 is interposed therebetween.

[0006] The operation of the conventional conductive cooling typecondensation particle counter constructed as such will be explained asfollows. First, air with the ultra-fine particles floating therein(hereinafter, referred to as “aerosol”) is supplied into the saturator20, which is kept at the temperature of 35° C. by the heater 26, throughan inlet 24 of the saturator 20, and then, it is saturated with thealcohol 12. The alcohol-saturated air continues to flow downstream, andit passes through the condenser 30 corresponding to a cold region ofwhich temperature is maintained at 10° C. The alcohol-saturated airpassing through the condenser 30 is supersaturated, and the alcohol isthen condensed around the particles in the air so that the particlesbecome grown. The grown particles become larger to their sizes of about12 μm and are then discharged from the condenser 30. Thus, the opticalinstrument 50 can readily measure the number of the particles.Furthermore, the grown particles are sucked by the vacuum pump 70, andthe flow rate of the particles sucked into the vacuum pump 70 isregulated by the flowmeter 60.

[0007] In case of the third type or conductive cooling type condensationparticle counter, the thermo-electric cooler used for keeping thecondenser at the low temperature of 10° C. is poor in view of acoefficient of performance for removing heat from the condenser by usingelectricity, and thus, the cooler is good for removing a small quantityof heat from the condenser but inappropriate for removing a largequantity of heat from the condenser. Accordingly, a capacity of thecondensation particle counter used commonly and currently is about 0.3to 1.0 liter/min since it is difficult to cool down a large quantity ofair. In particular, in case of the semiconductor clean rooms where it isnecessary to sample the particles at a high flow rate, a need for acondensation particle counter capable of sampling the particles at thehigh flow rate has been required for a long time. However, suitableequipment has not yet been developed. In addition, if the water is usedas an operating liquid, the pure ultra-fine particles are merelydischarged toward the outside of the condenser since the water vaporpassing through the condenser is first condensed at an inner wallsurface of the low-temperature condenser without a condensing process inwhich the vapor is condensed around the particles serving as the nuclei.Thus, there is a problem in that the ultra-fine particles cannot bemeasured through the optical instrument. That is, the conductive coolingtype condensation particle counter has a disadvantage in that only thealcohol must be utilized as the operating liquid. In particular, sincethe alcohol becomes a pollution source, the conductive cooling typecondensation particle counter in which the alcohol should be used as theoperating liquid is not suitable for a semiconductor fabricationprocess.

DISCLOSURE OF INVENTION

[0008] An object of the present invention is to provide a condensationparticle counter capable of readily and quickly counting ultra-fineparticles floating in air.

[0009] Another object of the present invention is to provide acondensation particle counter in which various operating liquids can beemployed and which can be applied to semiconductor clean rooms where aclean state should be maintained without any contamination.

[0010] A further object of the present invention is to provide acondensation particle counter in which the number of particles can bemeasured at a high flow rate.

[0011] In order to achieve the aforementioned objects, the condensationparticle counter of the present invention comprises a storage pool withan operating liquid contained therein, a saturator integrally formedwith the storage pool and having an absorbing member provided therein tocome into contact with the operating liquid for absorbing the operatingliquid and a heater installed at an outer wall thereof for forming theoperating liquid into vapor, a capillary located downstream of thesaturator for condensing the vapor therein to grow particles, an opticalinstrument installed adjacent to an outlet of the capillary for countingthe grown particles, and a suction means located downstream of thecapillary for sucking the grown particles therein.

BRIEF DESCRIPTION OF DRAWINGS

[0012]FIG. 1 is a sectional view showing the constitution of aconventional condensation particle counter.

[0013]FIG. 2 is a sectional view showing the constitution of acondensation particle counter according to the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

[0014] Hereinafter, a preferred embodiment of a condensation particlecounter according to the present invention will be explained in detailwith reference to the accompanying drawings.

[0015]FIG. 2 is a sectional view showing the constitution of thecondensation particle counter according to the present invention. InFIG. 2, like elements, which are the same as those of the conventionalcondensation particle counter shown in FIG. 1, are denoted by likereference numerals.

[0016] An operating liquid 14 is contained in a storage pool 10, and acylindrical absorbing member 22 is attached to an inner wall of asaturator 20 integrally formed with the storage pool 10. In the presentinvention, various liquids including alcohol and water can be utilizedas the operating liquid. The operating liquid is absorbed into theabsorbing member 22 which is made of porous material such as nonwovenfabric and of which one end 22 a is immersed into the operating liquidwithin the storage pool 10, and thus, the other end 22 b of theabsorbing member is caused to be wetted. At an outer wall of thesaturator 20 is installed a heater 26 for heating the operating liquidpermeated into the absorbing member 22. The heater 26 heats up theoperating liquid either to about 35° C. in a case where the operatingliquid is alcohol or to about 70° C. in a case where the operatingliquid is water.

[0017] A capillary 40 is located downstream of the saturator 20, and aninsulating member 42 is installed at an outer wall of the capillary 40in order to shut out heat flow between the capillary and theenvironment. In the capillary 40, vapor of the operating liquid iscondensed and the particles grow. If supersonic flow is accomplished inthe capillary 40, a mass flow rate of the vapor including the particlesbecomes constant. In such a case, the flowmeter 60 provided in theconventional condensation particle counter shown in FIG. 1 is notrequired. In order to sense and count the grown particles, a well-knownoptical instrument 50 is located in the vicinity of a leading end of thecapillary 40.

[0018] A vacuum pump 70 for sucking the grown particles therein isinstalled downstream of the capillary 40 in a state where a pipe 62 isinterposed therebetween. A well-known pressure gauge 64 for detectingsufficient pressure drop is installed at the pipe 62. If the pressurewithin the capillary 40 is reduced sufficiently to 0.5 atm or less bymeans of the vacuum pump 70, the supersonic flow becomes accomplished inthe capillary 40. If the pressure within the capillary 40 is maintainedat 0.5 atm or less by means of the vacuum pump 70, vapor temperaturewithin the capillary 40 is greatly lowered through adiabatic expansionof vapor so that the vapor is maintained at a supersaturated state.Thus, the vapor is condensed around the particles serving as the nuclei,and then, the particles become grown.

[0019] Hereinafter, the operation of the condensation particle counteraccording to the present invention will be explained. First, aerosol issupplied into the saturator 20, which is maintained at a predeterminedtemperature by the heater 26, through an inlet 24 of the saturator.Then, the air is saturated with the operating liquid such as the alcoholor water. The air saturated with the operating liquid continues to flowdownstream, and it passes through the capillary 40. The air, which hasbeen saturated with the operating liquid and passes through thecapillary 40, is supersaturated by an operation of the vacuum pump 70,and the vapor is then condensed around the particles so that theparticles become grown. As described above, a growing process of theparticles is accomplished in such a manner that when the vacuum pump 70causes the capillary to be kept at a very low pressure, the temperatureof the vapor is greatly lowered through the adiabatic expansion of thevapor within the capillary so that the vapor is caused to be kept at thesupersaturated state. At this time, if the supersonic flow of the vaporis also accomplished in the capillary 40, the mass flow rate of thevapor including the particles becomes constant. The particles grown byvapor condensation become larger and are then discharged from thecapillary 40. Thus, the optical instrument 50 can readily count thenumber of the particles. Then, the grown particles are sucked by thevacuum pump 70.

[0020] Although the invention has been described with respect to thepreferred embodiment, the scope of protection sought in the presentinvention is not limited thereto. It will be understood by the skilledin the art that specific design and constitution described in thepreferred embodiment is one example of the present invention and thatvarious changes and modifications may be made thereto without departingfrom the spirit and scope of the invention as defined in the appendedclaims.

[0021] Industrial Applicability

[0022] As described above, the condensation particle counter of thepresent invention can quickly and readily count the ultra-fine particlesfloating in the air, and various operating liquids such as water as wellas alcohol can also be utilized in the condensation particle counter.Furthermore, the condensation particle counter can be applied even tothe semiconductor clean rooms. Finally, the number of particles can bemeasured at a high flow rate.

1. A condensation particle counter, comprising: a storage pool with anoperating liquid contained therein; a saturator integrally formed withthe storage pool and having an absorbing member provided therein to comeinto contact with the operating liquid for absorbing the operatingliquid and a heater installed at an outer wall thereof for forming theoperating liquid into vapor; a capillary located downstream of thesaturator for condensing the vapor therein to grow particles; an opticalinstrument installed adjacent to an outlet of the capillary for countingthe grown particles; and a suction means located downstream of thecapillary for sucking the grown particles thereinto.
 2. The condensationparticle counter as claimed in claim 1, further comprising an insulatingmember for insulating the capillary from the environment.
 3. Thecondensation particle counter as claimed in claim 1, wherein the suctionmeans is a vacuum pump.